The present invention relates to an image forming method in which an electrostatic latent image is developed and transferred onto an intermediate transfer member and the image is transferred and fixed onto a recording medium through heating in an electrophotographic method or an electrostatic recording method, and an image forming apparatus used in this method.
In an image forming apparatus of an ordinary electrophotographic system, for example, an image forming apparatus in which an electrostatic latent image is formed on an image bearing member and developed with a dry toner to form a toner image and the toner image is then electrostatically transferred and fixed onto a recording medium to obtain the image, there arise often problems that non-uniform density occurs in the image or a toner powder is scattered to impair the resolution of the image or the dot reproducibility. The non-uniform density or the toner scattering mainly occurs while the toner image on the image bearing member is electrostatically transferred onto the recording medium.
In the electrostatic transfer method, a toner transfer efficiency is increased in proportion to an intensity of an electric field to be applied to the toner layer. However, when the intensity of the electric field becomes higher than a certain degree, so-called Paschen discharge occurs to decrease the transfer efficiency. That is, the transfer efficiency shows a peak value in a certain intensity of the electric field. Generally, the peak value of the transfer efficiency, in many cases, does not reach 100%, and remains approximately 95% at the highest.
The transfer efficiency of the toner layer thus depends on the intensity of the electric field. Accordingly, as the intensity of the electric field is changed owing to a non-uniform thickness of the toner layer, or an uneven surface or non-uniform electric properties of a recording medium such as paper, the transfer efficiency is also changed. When the toner image formed on the recording medium is monochromic and a layer is thin, the intensity of the electric field is mainly changed owing to the uneven surface or the non-uniform electric properties of the recording medium, with the result that the image mottle occurs. When monochromic toner images formed independently on the image bearing member are overlaid and transferred onto the recording medium to form a color image, the image mottle also occurs owing to the uneven surface or the non-uniform electric properties of the recording medium. In the electrostatic transfer method, a difference in transfer efficiency by the change in a thickness of a layer between a portion of a high layer thickness formed by overlaying and transferring plural toner images and a portion of a low layer thickness formed by transferring a monochromic toner image is small, but the transfer efficiency is greatly changed and the image mottle tends to occur owing to the uneven surface or the non-uniform electric properties of the recording medium.
Meanwhile, in a so-called color image forming apparatus of an intermediate transfer system in which plural toner images formed independently on an image bearing member are electrostatically transferred primarily onto an intermediate transfer member having a less uneven surface with less non-uniform electric properties to be overlaid thereon in order and the multicolor toner image formed on the intermediate transfer member is secondarily transferred onto a recording medium, the transfer efficiency is less changed, so that an image with less image mottle can be obtained.
For transferring a toner image onto a recording medium such as paper electrostatically uniformly, it is required to apply a fixed electric field. With respect to a multicolor toner image formed on an intermediate transfer member, there is an area in which a toner image with plural layers, such as three or more layers is formed, while there is an area in which no toner image layer is formed. Accordingly, it is difficult to apply a fixed electric field to the toner image of which the layer thickness varies greatly, and the intensity of the electric field tends to be non-uniform. Consequently, in the secondary transfer by the electrostatic transfer method, not all of the multicolor toner images formed on the intermediate transfer member are transferred onto the recording medium, and a part thereof remain on the intermediate transfer member. The amount of the toner remaining on the intermediate transfer member varies depending on the thickness of the toner layer on the intermediate transfer member. As a result, the color balance of the color images obtained on the recording medium is lost, and desired color images are hardly obtained. Besides, due to the uneven surface of the recording medium, the recording medium and the intermediate transfer member are not completely adhered. The transfer electric field becomes non-uniform owing to a non-uniform gap generated therebetween, or the transfer efficiency is decreased with a Coulomb repulsion force of toners to decrease the image quality.
In order to solve these problems, Japanese Patent Publication No. 41679/1971 discloses an image forming method which has steps of adhesively transferring a toner image formed on an image bearing member onto a surface of an elastic intermediate transfer member, then heating a recording medium fed between the intermediate transfer member and a heating roller using the heating roller, and fusing the toner image on the intermediate transfer member to thermally transfer the toner image on the transfer member onto the recording medium. Further, Japanese Patent Publication Nos. 1024/1989 and 1027/1989 disclose a method in which an endless belt-like intermediate transfer member and a recording medium superposed with a toner image transferred onto the intermediate transfer member therebetween are urged with a heating roll and a pressure roll to transfer and fix the toner image on the intermediate transfer member onto a recording medium. Still further, Japanese Patent Publication Nos. 20632/1982, 36341/1983 and 1023/1989 disclose a method which has steps of heating a toner image transferred onto an endless belt-like intermediate transfer member to a temperature above a melting point of a toner, and then urging the intermediate transfer member against a recording medium to transfer and fix the toner image on the intermediate transfer member onto the recording medium, wherein after the intermediate transfer member is urged against the recording medium, the intermediate transfer member and the recording medium are circulated and moved while being contacted with each other for a long period of time, and heat transfer from the intermediate transfer member to the recording medium is satisfactorily conducted in this contact state to surely transfer and fix the toner image on the intermediate transfer medium onto the recording medium.
In these non-electrostatic transfer methods, the troubles caused by the non-uniformity of the electric field which are found in the foregoing electrostatic transfer method do not occur, so that a high-quality image with a good color balance can be obtained in a color image with a high transfer efficiency of a toner image and a high sharpness. However, in the methods disclosed in Japanese Patent Publication Nos. 41679/1989, 1024/1989 and 1027/1989, there are problems that since a pressure roll mounted on the reverse side of the recording medium is not provided with a heating unit, the recording medium takes out a large amount of heat so that the toner of the toner image in contact with the recording medium is hardly fused on the recording medium and insufficient fixing tends to occur in the image formation at a high speed in particular.
Furthermore, in the methods disclosed in Japanese Patent Publication Nos. 20632/1982, 36341/1983 and 1023/1989, there are problems that while the intermediate transfer member and the recording medium are moved in contact with each other for a long period of time, they come sometimes out of contact with each other, and therefore image disorder occurs or a pressure applied to the intermediate transfer member and the recording medium becomes non-uniform to cause image disarray.
Besides these methods, Japanese Patent Publication Nos. 63756/1991, 63757/1991 and 63758/1991 disclose a transfer and fixing method wherein in an image forming apparatus in which an intermediate transfer member carrying a toner image is urged against a recording medium with a pair of pressure rolls to transfer and fix the toner image on the intermediate transfer member onto the recording medium, a heater for preheating the recording medium is, separately from the pair of pressure rolls, mounted on an upstream side of a transfer and fixing zone to enable the high-speed fixing. Among the transfer-fixing methods using the heater for preheating as disclosed in these three documents, the method disclosed in Japanese Patent Publication No. 63756/1991 is a method in which the toner image on the intermediate transfer member is heated at a temperature lower than the fusing temperature of the toner, the pressure rolls heated at a temperature higher than the fusing temperature of the toner is urged against the intermediate transfer member, and the recording medium heated at the temperature higher than the fusing temperature of the toner is fed to the urged portion to transfer and fix the toner image onto the recording medium.
The method disclosed in Japanese Patent Publication No. 63757/1991 is, unlike the method of Japanese Patent Publication No. 63756/1991, a method in which the toner image on the intermediate transfer member is heated to a temperature lower than the fusing temperature of the toner, the pressure rolls heated at a temperature lower than the fusing temperature of the toner is urged against the intermediate transfer member, and the recording medium heated to a temperature higher than the fusing temperature of the toner is fed to the urged portion to transfer and fix the toner image onto the recording medium. Further, the method disclosed in Japanese Patent Publication No. 63758/1991 is, unlike the methods of Japanese Patent Publication Nos. 63756/1991 and 63757/1991, a method in which the toner image on the intermediate transfer member is heated to a temperature lower than the fusing temperature of the toner, the pressure rolls heated to a temperature higher than the fusing temperature of the toner are urged against the intermediate transfer member, and the recording medium heated at a temperature lower than the fusing temperature of the toner is fed to the urged portion to transfer and fix the toner image onto the recording medium.
In the transfer and fixing method using the heater for preheating as disclosed in these three documents, the excessive heating of the pressure rolls can be controlled to improve the thermal efficiency. However, it is difficult to completely eliminate the non-uniform melting of the toner image.
A fixing method and a fixing unit having a pair of pressure members and a pressure member heater that heats the pressure members as employed in an image forming apparatus are described in, for example, Japanese Patent Publication No. 4699/1984 and Japanese Patent Laid-Open Nos. 74579/1984 and 129768/1985. In the fixing method and the fixing unit, a main part has a rotatable heat-fixing roll having a heating source therein, a rotatable pressure roll mounted by being urged against the heat-fixing roll and a release agent feeding unit mounted on the heat-fixing roll to feed a release agent for preventing offset to the outer periphery of the heat-fixing roll, and a transfer paper that carries an unfixed toner image is passed between the heat-fixing roll and the pressure roll to fix the toner image. The heat-fixing roll includes a substrate roll having a heating source therein, an inner elastic layer formed on the substrate roll and an outer elastic layer mounted on the inner elastic layer and formed of an elastic material having an affinity for the release agent for preventing offset and an abrasion resistance, such as a fluororubber. The heat-fixing roll is brought into contact with the transfer paper by the elasticity of the inner elastic layer with an appropriate pressure and an appropriate contact width, and the offset phenomenon is prevented with the action of the release agent fed to the outer elastic layer.
Moreover, to meet the high speed, a method using a belt is proposed as described in Japanese Patent Laid-Open No. 132972/1986 (this method is hereinafter referred to as a belt nip method). In the belt nip method, using a fixing unit having an endless belt rotatably tensioned with plural support rolls and a heat-fixing roll that forms a belt nip in contact with the endless belt, a paper having an unfixed toner image formed thereon is passed through a belt nip between the heat-fixing roll and the endless belt to fix the image with the pressure and the heat energy in the belt nip. After passed through the belt nip, the paper is peeled off with a peel nail, and discharged outside the fixing unit. In this construction, the greater width of the belt nip between the endless belt and the heat-fixing roll can easily be secured than in the ordinary roll nip method to cope with the high speed. Further, at the same fixing speed, the heat-fixing roll in the belt nip method can be downsized in comparison with that in the roll nip method.
Nevertheless, a so-called offset phenomenon tends to occur that when the surface of the heat-fixing roll is contacted with the toner surface, the toner fused is adhered to the surface of the heat-fixing roll and migrates to a transfer medium such as paper to be fed later. In order to prevent the offset phenomenon, the surface of the heat-fixing roll is coated with a material having a good releasability from the toner fused, such as a silicone rubber or a fluororesin or with a liquid release agent such as silicone oil.
On the other hand, in recent years, an electrophotographic process has found wide acceptance in not only copying machines but also printers because of the development of appliances or the improvement of communication network in society of information technology, and downsizing, weight reduction, high speed and reliability of apparatus used have been increasingly required strictly. Especially in case of color electrophotography, an image formed is required to have a high quality and a high level of color formation. For obtaining a high-quality image with a high level of color formation, it is required, in view of a light transmission and a gloss, that a toner is satisfactorily fused and a surface of an image after fixed is smooth. To this end, a fixing step in the electrophotographic process is especially important.
As a contact-type fixing method which has been often used, a method using a heat and a pressure in the fixing (hereinafter referred to as a heat-pressing method) is generally employed. In case of the heat-pressing method, a surface of a fixing member and a toner image on a transfer medium are contacted under pressure. Accordingly, a thermal efficiency is quite good, and the fixing can quickly be conducted. This method is quite effective in a high-speed electrophotographic copying machine.
However, since the surface of the fixing member is contacted with the toner image under pressure in a heat-fused state in the heat-pressing method, an offset or wrapping phenomenon in which a part of the toner image migrates to the surface of the fixing member by being adhered thereto is liable to occur. In particular, in the color toner fixing in which plural color toners have to be fused and mixed, it is required, in comparison with the monochromic toner fixing, that sufficient heat and pressure are applied to the toner to make the toner flowable and that a toner layer in a fused state which is thick with plural colors overlaid is released without an offset or wrapping phenomenon. Thus, the releasing in the fixing of the color toner is more difficult than that in the fixing of the monochromic toner.
With respect to a simple method for preventing the adhesion of the toner to the surface of the fixing member, the surface of the fixing member is coated with silicon oil as a liquid for preventing offset. However, the use of oil involves a problem of adhesion of oil to the transfer medium and the image after the fixing. Further, it is problematic in that a tank for storing oil is required in the fixing unit which makes it difficult to downsize the fixing unit and that supply of oil is troublesome to restrict the cost reduction.
Ordinarily, the amount of oil coated on a general transfer medium in the color fixing is as large as approximately 8.0xc3x9710xe2x88x922 mg/cm2, while oil is not used at all in monochromic printers or even when oil is used, its amount is less than 8.0xc3x9710xe2x88x924 mg/cm2 which is {fraction (1/100)} of the amount of oil coated in the color fixing. Thus, the foregoing defect is not given in practice. Thus, it has been earnestly demanded that even the color fixing is enabled with the same amount of oil as in monochromic printers. Accordingly, various methods have been proposed in which the releasability of the toner is improved not by a fixing unit but by modification of a toner resin or a wax.
For example, Japanese Patent Laid-Open No. 158340/1981 discloses a monochromic toner that exhibits an excellent oilless fixing suitability by effects of a resin containing a low-molecular component and a high-molecular component and thus having a wide molecular weight distribution and a wax. The resin for the monochromic toner is adapted to endure a peel strength exerted on a toner layer in an interface of a fixing unit, namely to prevent offset with an elasticity of a rubber given by entanglement of the high-molecular component diluted with the low-molecular component.
However, when this technique is developed in the fixing of a color image, there are some problems. That is, (1) since the binder resin having the rubber elasticity given by entanglement of the high-molecular component is used, a gloss level of an image fixed is lowered to decrease color formation of a color image. (2) The binder resin is elastic but is itself soft and liable to deformation because it contains the low-molecular component in the molecule. Accordingly, when the number in toner layer is increased and 3 or 4 layers are used as in a color image, the binder resin tends to cause wrapping of the toner layers around a fixing unit in deformation by peeling to decrease a peelability. (3) In case of a color image with multiple toner layers, a wax is bled out between toner layers having different colors, with the result that the peeling of the toner layers, namely the offset tends to occur. Thus, the effect of preventing the offset is not so obtained as in the fixing of the monochromic image.
In the color toner as well, various fixing units such as a fixing unit using a high-molecular component and a fixing unit using a wax have been proposed. It is however difficult to overcome the foregoing problems. Even though a releasability is somewhat improved, the improvement with no practical problem by using oil in the same amount as in the fixing of the monochromic toner has not yet been attained.
Moreover, when the wrapping of the toner layer around the fixing unit by the adhesion of the toner can be prevented, a hot offset resistance is obtained by the viscoelasticity properties though somewhat controlling the color formation. However, in the resin of which the molecular weight distribution is widened using the mere combination of the high-molecular component and the low-molecular component, no sufficient releasability is obtained, and the large amount of oil is therefore needed for preventing the wrapping of the toner layer as stated above. Further, a styrene-acrylic resin tends to cause wrapping phenomenon around a fixing unit because of a low elastic modulus of a rubber due to a resin composition, even though a molecular weight is increased. Thus, no sufficient peelability is provided.
In addition, a fixing unit using a high-molecular component or a toner with a wax is problematic in that a gloss level is decreased. Especially when a ratio of a high-molecular component is increased, a gloss level is extremely decreased. This cannot be controlled by increasing a fixing temperature, and it is ascribable to the material.
In the mechanism of heating the intermediate transfer member and the belt nip method, not all of toners can be used in view of controlling the gloss level. The heating of the intermediate transfer member is advantageous in that a high gloss level can be obtained regardless of a type of a toner material. However, when the intermediate transfer member is preheated to decrease image unevenness and obtain a transfer efficiency, an image tends to be disarrayed on the intermediate transfer member heated in case of, for example, a toner using a resin having a low glass transition point (Tg). Further, in case of a toner using a resin containing a large amount of a high-molecular component, there is a tendency that an excessive amount of electricity is required to fuse the toner. In the pressure fixing area also, a gloss level which is an important factor of an image quality is restricted by the type of the toner material in the belt nip method alone. For example, when a low-molecular resin is used, a high gloss level is provided. Meanwhile, when a high-molecular resin is used, a low gloss level is provided. It is thus difficult to control the gloss level.
The invention has been made in view of the foregoing circumstances, and provides an image forming method and an image forming apparatus. That is, the invention provides, upon solving the problems in the related art, an image forming method in which without substantially feeding a release agent, neither image disarray in image transfer nor non-uniform melting of a toner occurs and a gloss level of an image can be controlled, and an image forming apparatus used in this method.
According to an aspect of the invention, an image forming method has steps of: transferring a toner image formed on an image bearing member onto an intermediate transfer member; and simultaneously transferring and fixing the toner image on the intermediate transfer member onto a recording medium using a transfer and fixing unit. The toner forming the toner image contains a binder resin and a colorant, and the toner has a storage elastic modulus (Gxe2x80x2) of 2xc3x97102 to 6xc3x97103 Pa at a temperature at which a loss elastic modulus (Gxe2x80x3) of the toner reaches 1xc3x97104 Pa, and the transfer and fixing unit has a nip between a fixing roll coated with an elastic member and a heat-resistant belt laid across in a tensioned condition with support rolls, and the heat-resistant belt is urged against the fixing roll and the elastic member of the fixing roll is twisted at an exit of the nip with a pressure roll mounted inside the heat-resistant belt through the heat-resistant belt.
In the binder resin of the toner, a number average molecular weight (Mn) is in the range of 2,500 to 20,000, a weight average molecular weight (Mw) is in the range of 9,000 and 90,000, a softening point (Tm) is in the range of 60xc2x0 C. to 120xc2x0 C., and a glass transition point (Tg) is in the range of 45xc2x0 C. to 70xc2x0 C.
According to another aspect of the invention, an image forming apparatus has: a transfer unit that transfers a toner image formed on an image bearing member onto an intermediate transfer member; and a simultaneous transfer and fixing unit that transfers and fixes the toner image on the intermediate transfer member onto a recording medium. The toner forming the toner image contains a binder resin and a colorant, and the toner has a storage elastic modulus (Gxe2x80x2) of 2xc3x97102 to 6xc3x97103 Pa at a temperature at which a loss elastic modulus (Gxe2x80x3) of the toner reaches 1xc3x97104 Pa, and the transfer and fixing unit has a nip between a fixing roll coated with an elastic member and a heat-resistant belt laid across in a tensioned condition with support rolls, and the heat-resistant belt is urged against the fixing roll and the elastic member of the fixing roll is twisted at an exit of the nip with a pressure roll mounted inside the heat-resistant belt through the heat-resistant belt.